The following is a discussion of relevant art pertaining to RNAi. The discussion is provided only for understanding of the invention that follows. The summary is not an admission that any of the work described below is prior art to the claimed invention.
RNA interference refers to the process of sequence-specific post-transcriptional gene silencing in animals mediated by short interfering RNAs (siRNAs) (Zamore et al., 2000, Cell, 101, 25-33; Bass, 2000, Cell, 101, 235; Fire et al., 1998, Nature, 391, 806; Hamilton et al., 1999, Science, 286, 950-951; Lin et al., 1999, Nature, 402, 128-129; Sharp, 1999, genes & Dev., 13:139-141; and Strauss, 1999, Science, 286, 886). The corresponding process in plants (Heifetz et al., International PCT Publication No. WO 99/61631) is commonly referred to as post-transcriptional gene silencing or RNA silencing and is also referred to as quelling in fungi. The process of post-transcriptional gene silencing is thought to be an evolutionarily-conserved cellular defense mechanism used to prevent the expression of foreign genes and is commonly shared by diverse flora and phyla (Fire et al., 1999, Trends genet., 15, 358). Such protection from foreign gene expression may have evolved in response to the production of double-stranded RNAs (dsRNAs) derived from viral infection or from the random integration of transposon elements into a host genome via a cellular response that specifically destroys homologous single-stranded RNA or viral genomic RNA.
The therapeutic potential of RNAi lies in the ability to modulate gene expression in a sequence specific manner by harnessing a highly conserved, robust endogenous mechanism of action. This endogenous mechanism of action vastly expands upon the number of available targets for disease modification when compared to existing small molecule and biologic modalities. Nevertheless, a opposed to exogenously supplied small molecule and biologic modalities, the RNA molecules that serve as triggers for RNAi are not well suited for administration due to their inherent instability, especially in biologic systems. This problem has been addressed through innovation, both in terms of chemical modification of RNA triggers (see U.S. Ser. No. 10/444,853, published as US US.S Patent Appl. Publ. No. 20040192626) and various delivery approaches (see U.S. Ser. No. 11/586,102, published as US US-.S. Patent Appl. Publ. No. 20080020058)), which have provided compounds and compositions that may be available for clinical development. Nevertheless there remains a need for additional RNA triggers that are available to expand the repertoire of available compounds and compositions for use in RNAi based therapeutics, and especially compounds and compositions that are compatible with different delivery systems and/or routes of administration.